Various types of systems have been used to detect the presence of a particular chemical or molecule in a complex sample. For example, antibodies are used to detect the presence of a protein in a sample, and DNA microarray chips have been used to identify genes and study gene expression. Most existing molecular detection systems are designed to detect the presence of a single type or single category of target molecule. In the case of antibody detection, existing systems are typically limited to detecting only a subset of a type of molecule.
Recently, it has been shown that RNA and DNA aptamers can substitute for monoclonal antibodies in various applications (Jayasena, “Aptamers: an emerging class of molecules that rival antibodies in diagnostics.” Clin. Chem., 45(9):1628-50, 1999; Morris et al., “High affinity ligands from in vitro selection: complex targets.” Proc. Natl. Acad. Sci., USA, 95(6):2902-7, 1998). The relatively fast selection process of the specific aptamers and the inexpensive synthesis makes the aptamer useful alternatives for monoclonal antibodies. These nucleic acids can be easily synthesized, readily manipulated, and can be stored for over long time. These benefits make nucleic acids more attractive biotechnology tools than their counterpart of proteins, antibodies. Additionally these nucleic acid probes can also be labeled by radioisotope, biotin, or fluorescent tags and can be used to detect targets under various conditions. An increasing number of DNA and RNA aptamers that recognize their non-nucleic acid targets has been developed by SELEX and has been characterized (Gold et al., “Diversity of Oligonucleotide Functions,” Annu. Rev. Biochem., 64:763-97.1995; Bacher & Ellington, “Nucleic Acid Selection as a Tool for Drug Discovery,” Drug Discovery Today, 3(6):265-273, 1998).